1. Technical Field
The present invention relates, generally, to a power supply having a synchronous rectifier configured to synchronize the output duty cycle as a function of voltage on the secondary winding and, more particularly, to an integrated synchronous rectifier configured to derive the timing of polarity reversals from the secondary winding and to generate a current source for controlling the output rectifier synchronously with the detected polarity reversals.
2. Background Art and Technical Problems
Power supplies are known to employ switching type power converters to provide a step-down rectified voltage to an electrical load. Self-synchronized rectifiers generally refer to rectifiers using MOSFET rectifying devices having control gates which are themselves controlled by the voltages of the secondary winding of the power transformer in order to provide the rectification of the output of the transformer. Presently known self-synchronized rectifiers, however, do not adequately accommodate the large variability of transformer voltages in forward type converters. In particular, if the voltage level on the secondary winding in the forward mode is too low, the MOSFET rectifying devices may not always turn on when needed. Conversely, for excessively high voltage levels, at the output winding, correspondingly high control voltages applied to the MOSFET rectifiers can destroy the rectifier.
Several attempts have been made to address the shortcomings associated with presently known synchronous rectifiers. See, for example, Rozman, U.S. Pat. No. 5,528,482, issued Jun. 18, 1996, entitled "Low Loss Synchronous Rectifier for Application to Clamped-Mode Power Converters"; Barlage, U.S. Pat. No. 5,396,412, issued Mar. 7, 1995, entitled "Synchronous Rectification and /Adjustment of Regulator Output Voltage"; Smith, U.S. Pat. No. 5,304,875, issued Apr. 19, 1994, entitled "Efficient Transistor Drive Circuit for Electrical Power Converter Circuits and the Like"; Schwartz, U.S. Pat. No. 5,552,695, issued Sep. 3, 1996, entitled "Synchronous Rectified Buck-Flyback DC to DC Power Converter"; Gauen, U.S. Pat. No. 5,126,651, issued Jun. 30, 1992, entitled "Gate Drive Circuit for a Synchronous Rectifier"; Bowman, el al., U.S. Pat. No. 5,590,032, issued Dec. 31, 1996, entitled "Self-Synchronized Drive Circuit for a Synchronous Rectifier in a Clamped-Mode Power Converter"; and Fisher, et al., U.S. Pat. No. 5,179,512, issued Jan. 12, 1993, entitled "Gate Drive for Synchronous Rectifiers in Resonant Converters." The entire disclosures of these patents are hereby incorporated herein by this reference.
However, presently known synchronous rectifier designs remain inadequate, particularly in their ability to accommodate the variability in transformer voltages, propagation delay, and high switching frequencies. A synchronous rectifier design is thus needed which addresses the shortcomings of the prior art.